The present invention provides, for example, a method to reduce hum in a hearing aid due to RF electromagnetic compatibility (EMC) effects when a hearing aid wearer uses a cordless (e.g., cellular) phone employing intermittent transmission.
Conventional radiocommunication systems provide examples of three different types of radio transmission protocol for use by cellular or cordless phones. The earlier deployed systems, such as the U.S. AMPS system, the British TACS system and the Scandinavian NMT system, employ continuous analog FM transmission of speech, and each conversation is allocated a different channel. This technique is known as Frequency Division Multiple Access or FDMA.
More recently, digitally coded speech has been employed in systems such as the European GSM system, the U.S. IS-54 system and the Japanese PDC system, because it affords better protection against fading in the mobile radio propagation environment, higher system capacity, longer battery life and privacy through digital encryption. These systems employ Time Division Multiple Access (TDMA) techniques whereby a single radio frequency channel is shared between several calls by allocating different timeslots to different calls. Additionally, the transmit and receive timeslots in these TDMA systems are staggered so that the phone does not have to transmit and receive simultaneously. This feature is advantageous in eliminating the large, expensive and lossy duplexing filters that were used with the earlier continuous analog transmission systems that required simultaneous transmission and reception. Moreover, time duplex is essential when the frequency band allocated for a system does not allow an adequate frequency spacing between transmit and receive, as-in the so-called U.S. "part 15, unlicensed bands", the bands used for the European digital cordless phone standard known as DECT, and in the 1900 MHz PCS bands just released in the USA.
In time duplex operation, the transmitter is inactive for a certain period during each frame, which period is at least long enough to receive a signal burst. The loss of transmission time caused by this inactive period is made up by buffering the digitized speech signal to be transmitted in a memory and subsequently transmitting the buffered signal at a higher rate than that at which it was buffered during its allocated slot. The peak transmitter power is increased by the same factor to support the higher rate, maintaining the same (or lower) mean power. When such a phone is operated near sound reproducing equipment that is not designed for operation in close proximity to radio transmitters, such as hearing aids or high-fi equipment, the transmitter pulse repetition rate can be spuriously detected in the equipment giving rise to an audible buzz. This effect only occurs when the cordless phone is extremely close to such equipment, for example, within three feet or less, which is the case when a phone is placed next to the hearing aid of a hearing impaired subscriber.
A third access method that has been proposed is Code Division Multiple Access (CDMA). CDMA systems may employ continuous transmission but still permit many conversations to take place using the same spectrum, as the differently coded signals can overlap without interference providing their relative power levels are not too divergent. A disadvantage of CDMA phones is their lack of retrospective compatibility with earlier standards, an important factor in introducing new systems, or the larger size and higher cost of providing a dual mode phone with retrospective compatibility. CDMA systems (such as that described in the US IS-95 standard) are also risky precisely because of their need for very strict power control since one errant phone that incorrectly controls its power can jam the whole system. A touted potential advantage of CDMA systems generally is that the phone transmits a lower power continuously instead of using higher power bursts, however the U.S. IS-95 system employs burst transmission of varying duty factor in the phone and thus does not offer much improved hearing aid compatibility.
Hybrid time duplex CDMA systems have been developed to overcome the disadvantages of IS-95, such as the Omnipoint wideband CDMA system developed for use in the so-called "unlicensed" bands that do not have a frequency duplex spacing. Time-duplex CDMA phones also employ intermittent or burst transmission and so require a method to improve compatibility with the use of hearing aids.
Since the above-mentioned TDMA systems account for over 99% of the current world market, it is important to provide phones which also include an operating mode of improved hearing aid compatibility so that hearing aid wearers are not subjected to the above-described annoying hum or buzz.